Editing Programmable logic controller (section)

=== Communication ===
PLCs use built-in ports, such as [[USB]], [[Ethernet]], [[RS-232]], [[RS-485]], or [[RS-422]] to communicate with external devices (sensors, actuators) and systems ([[programming software]], [[SCADA]], [[user interface]]). Communication is carried over various industrial network protocols, like [[Modbus]], or [[EtherNet/IP]]. Many of these protocols are vendor specific.

PLCs used in larger I/O systems may have [[peer-to-peer]] (P2P) communication between processors. This allows separate parts of a complex process to have individual control while allowing the subsystems to co-ordinate over the communication link. These communication links are also often used for user interface devices such as keypads or [[Personal computer|PC]]-type workstations.

Formerly, some manufacturers offered dedicated communication modules as an add-on function where the processor had no network connection built-in.